1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery capable of automatically stopping charging or discharging when swelling occurs for improved safety.
2. Description of the Related Art
As generally known in the art, the internal temperature of a secondary battery (for example, a lithium ion secondary battery) tends to rise when the battery is overcharged to approximately 4.5V or more. Such a tendency has been used to develop a safety device for interrupting the overcharging of secondary batteries, and a typical example thereof is a Positive Temperature Coefficient (PTC) thermistor which is serially connected to a large-current path for charging/discharging. The PTC thermistor is a safety device adapted to reduce or completely interrupt the charging/discharging current of a secondary battery by increasing the resistance value as temperature rises.
The PTC thermistor is generally positioned on a surface of a case to be actuated in direct response to the temperature of the case. An end of the PTC thermistor is welded to the case via a lead and the other end thereof is electrically connected to a protective circuit module via another lead.
When the internal temperature of the secondary battery rises due to overcharging, the electrolyte decomposes and generates gas, and the internal pressure increases. This leads to “swelling” wherein the case of the secondary battery swells outward. Of course, safety measures have been devised against such swelling. For example, the case of the secondary battery can have a safety vent formed on the surface thereof with a smaller thickness, which is adapted to fracture and discharge the internal gas to the exterior when the internal pressure is above a predetermined value or the swelling thickness exceeds an allowable value. Of course, the quality of safety provided by the safety vent is not very good, because the electrolyte is discharged to the exterior. However, such damage is believed to be more tolerable than an explosion or burning of the secondary battery, and this is why the safety vent is currently used.
The swelling is caused by various factors. For example, it results from over-discharging, an internal short circuit, an external short circuit, and external heat, in addition to the above-mentioned overcharging. As recent secondary batteries have a thinner case for improved capacity, the swelling tends to occur more frequently.
However, conventional PTC thermistors are actuated only in response to temperature, and not to the above-mentioned swelling, and have many problems as follows:
Firstly, conventional PTC thermistors must have different resistance values according to the capacity of each battery. For example, when a conventional PTC thermistor is used for a battery having a small capacity, it must be cut off at a low current value and must have a large resistance value. Alternatively, when a conventional PTC thermistor is used for a battery having a large capacity, it must be cut off at a high current value and must have a small resistance value. Considering that batteries having various capacities are manufactured, it is not easy to set the resistance value of PTC thermistors according to the capacity of respective batteries.
Secondly, conventional PTC thermistors have a difficulty in quickly suppressing the swelling caused by abruptly rising internal temperature of batteries for structural reasons. For example, the internal temperature of a battery can abruptly rise in an unexpected manner due to overcharging or an external short circuit. Of course, such an abrupt rise in temperature decomposes the electrolyte of the battery, generates a large amount of gas, and causes the swelling to proceed very rapidly. Although a conventional PTC thermistor would be actuated by such an abrupt rise in temperature, the degree of swelling can have exceeded an allowable value. In that case, the safety vent of the case of the battery is actuated and the internal electrolyte leaks, in spite of the actuation of the PTC thermistor, or the case can even explode or burn.
Thirdly, all batteries generally have a predetermined allowable swelling thickness and, when they swell beyond the predetermined thickness, the safety vent is normally actuated or the case explodes and burns. However, conventional PTC thermistors have a resistance value related only to temperature and, even when the batteries swell beyond the allowable thickness, are not actuated but still allow charging/discharging currents to flow as long as the temperature is below an allowable value. As such, conventional PTC thermistors cannot effectively suppress excessive swelling when the temperature is below an allowable level.
Fourthly, conventional PTC thermistors are connected to a conductive case via a lead. However, the lead and the case are not easily welded to each other. As a dangerous process of welding the lead to the case is added, defects are more likely to happen. In addition, a conductive plate of different material must be welded to the case beforehand according to the prior art, in order to improve the welding between the lead and the case. This degrades workability.
Fifthly, conventional PTC thermistors are serially connected to a large-current path for charging/discharging. Therefore, the PTC thermistors themselves consume, for example, discharging currents and shorten the overall battery capacity and service time.